Refine plugin guidance in AI system prompt

[nerrad]

Related issues

• Related to plugin site building improvements.

![NOTE]
Early testing is showing promising results for creating a commerce type site (inferring WooCommerce, and ensuring relevant block themes and blocks are used. But I'm still testing/tweaking because it's not quite there yet.

How AI was used in this PR

• AI-assisted authoring of the prompt guidance and PR preparation. Update this section if you want a more specific summary.

Proposed Changes

• Add guidance to discover and incorporate plugin-provided pages, templates, patterns, routes, and blocks into generated site designs.
• Prefer block-based plugin integrations and extending existing plugin blocks before creating new custom blocks.
• Require default plugin configuration and visible frontend integration so plugin installs produce a usable baseline experience.

Testing Instructions

• Review the prompt updates in apps/cli/ai/system-prompt.ts.
• Run a plugin-oriented site generation flow and confirm the agent uses relevant plugin-provided frontend experiences.
• Not run locally during this PR creation pass.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?